Method to measure and characterize interference throughout a cellular system

ABSTRACT

A computer implemented process compares signals communicated between a known position and a plurality of base stations in a cellular telephone system to determine the level of interference with a signal on a channel expected to serve the known position, and determines a value indicating a probability of interference with a signal on a channel expected to serve the known position.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to cellular telephone systems and, moreparticularly, to processes for designing and improving the performanceof cellular telephone systems.

[0003] 2. History of the Prior Art

[0004] Presently available commercial mobile communication systemstypically include a plurality of fixed base stations (cells) each ofwhich transmits signals to and receives signals from mobile units withinits communication area. Each base station is assigned a plurality ofchannels over which it can communicate with mobile units. A mobile unitwithin range of the base station communicates with the external worldthrough the base station using these channels. Typically, the channelsused by a base station are separated from one another sufficiently thatsignals on any channel do not interfere with signals on another channelused by that base station. To accomplish this, an operator typicallyallots to a base station a group of channels which are each widelyseparated from the next. So long as a mobile unit is within the area inwhich the signal from a base station is strong enough and iscommunicating with only that base station, there is no interference withthe communication.

[0005] In order to allow mobile units to transmit and receive telephonecommunications as the units travel over a wide geographic area, eachcell is normally physically positioned so that its area of coverage isadjacent to and overlaps the areas of coverage of a number of othercells. When a mobile unit moves from an area covered by one base stationto that covered by another, communication with the mobile unit istransferred (handed off) from one base station to another in an areawhere the coverage from different cells overlaps. Because of thisoverlapping coverage, the channels allotted to the cells are carefullyselected so that adjoining cells do not transmit or receive on the samechannels. The channels used by adjoining base stations are alsotheoretically separated from the channels of each adjoining base stationsufficiently that signals from any base station do not interfere withsignals from another adjoining base station. This separation istypically accomplished by assigning a group of widely separatednon-interfering channels to some central cell and then assigning othergroups of widely separated non-interfering channels to the cellssurrounding that central cell using a pattern which does not reuse thesame channels for the cells surrounding the central cell. The pattern ofchannel assignments continues similarly in the other cells adjoining thefirst group of cells. The pattern is often called a channel reusepattern.

[0006] So long as a mobile unit is within the area in which the signalfrom a base station is strong enough and is communicating with only thatbase station, there is no interference with the communications. However,when a mobile unit moves from an area covered by one base station tothat covered by another base station, the communication must betransferred from one base station to the other in an area. This requirescell coverage to overlap. Because of this overlapping coverage, thechannels allotted to the cells are carefully selected so that adjoiningcells do not transmit or receive on the same channels.

[0007] There are a number of different types of mobile communicationssystems. Channels are defined in different manners in each of thedifferent systems. In the most prevalent American Mobile Phone System(AMPS) system, channels are defined by frequency. A frequency band of 25MHz providing approximately four hundred different adjoining FMfrequency channels is allotted by the federal government to eachcellular operator. In a typical AMPS system, each channel uses a fixedFM frequency band width of 30 KHz. for downlink transmission from a basestation to a mobile unit and another fixed FM frequency band width of 30KHz. for uplink transmission from a mobile unit to a cell. Typically,the frequencies assigned to the downlink transmissions for an entirecellular system immediately adjoin one another and are widely separatedfrom the frequencies assigned to the uplink transmissions which alsoimmediately adjoin one another. In this specification, even thoughwidely separated, the pair of frequencies used for both downlink anduplink transmission are generally intended when reference is made to anAMPS channel unless the context indicates otherwise.

[0008] Since channels are defined by frequency in an AMPS system, thechannels used by any single base station are separated from one anotherin frequency sufficiently to eliminate interference between thosechannels. An operator typically allots a base station a set of channelswith frequencies which are each separated from the next by some largenumber (e.g., twenty-one) channels carrying intermediate frequencies.Thus, in a system with twenty-one channel separation, one base stationmight use channels 1, 22, 43, 64, 85, and so on up to a total of betweenfive and one hundred individual channels.

[0009] When a mobile unit moves from an area covered by one base stationto that covered by another base station in an AMPS system, thecommunication must be transferred from one base station to the other inan area in which cell coverage overlaps. Because of this overlappingcoverage, the channels allotted to the cells are carefully selected sothat adjoining cells do not transmit or receive on the same frequencies.This is typically accomplished by assigning channels to a central cellwhich are widely separated in frequency in the manner described above,and then assigning channels to the cells surrounding that central cellusing a pattern which increases each channel number by one for eachsequential cell surrounding the central cell. Thus, if cells arearranged in a honeycomb pattern in which six cells surround a centralcell using the above-described channels, a first cell adjacent to thecentral cell may have channels 2, 23, 44, 65, 86, and so on while asecond cell adjoining the central cell may have channels 3, 24, 45, 66,87, and so on. The pattern of channel assignments continues similarly inthe other cells adjoining the central cell.

[0010] In some AMPS systems, especially those with cells in urban areascarrying heavy traffic, each cell may be further divided into two orthree sectors each of which may include channels having theabove-described frequency allotment of channels. The antennas of eachsector are typically arranged to provide 180 or 120 degree coverage.When cells are discussed herein, sectors are normally meant as wellunless the context indicates otherwise.

[0011] Another type of mobile system called Code Division MultipleAccess (CDMA) uses digital signals to transmit data. All of the basestations of a CDMA system use the same “spread spectrum” frequency bandof 1.25 megacycles to transmit the digital signals. The transmissionsare combined with redundant channel coding information to allow errorcorrection. The encoded signals are then multiplied by one of sixty-fourWalsh codes which establish individual channels and increase thebandwidth to 1.25 megacycles. Because of the redundancy of the encodedsignals, a receiver may decode a signal from the plethora of codedchannels carrying data on the broad frequency band. Since the Walshcodes establish a number of individual channels and the pseudo-noisecode assigned to each base station differs from those of othersurrounding base stations, adjacent and remote cells may reuse the samefrequency bands.

[0012] In another common type of mobile system called Time DivisionMultiple Access (TDMA), frequencies are assigned to the entire system ingroups much like they are assigned in an AMPS system. However, withinany frequency, each base station sends and receives in bursts duringsome number of different intervals or time slots. These time intervalswithin frequency bands then effectively constitute the individualchannels. By assuring that the group of frequencies assigned to anyindividual base station differ from one another and from the frequenciesassigned to base stations surrounding each individual base station, achannel reuse pattern is established which allows substantially greateruse of the frequency spectrum because of the time division process.

[0013] In theory, these forms of cell arrangement and channelassignments allows channel reuse patterns to be repeated at distancesseparated sufficiently to negate interference between mobile units onthe same and adjacent channels.

[0014] Unfortunately, interference does occur for a number reasons.Antenna patterns, power levels, scattering, and wave diffraction differfrom cell to cell. Buildings, various other structures, hills,mountains, foliage, and other physical objects cause signal strength tovary over the region covered by a cell. Consequently, the boundaries atwhich the signal strength of a channel falls below a level sufficient tosupport communications with a mobile unit vary widely within a cell andfrom cell to cell. For this reason, cells adjacent one another do not,in fact, typically form the precise geometric boundaries suggestedabove. Since cell boundaries must overlap to provide complete coverageof an area and allow handoff and because the boundaries of cells areimprecisely defined, signals will often interfere with one another eventhough they are generated by cells which are at distances theoreticallysufficient to eliminate interference. This is especially true when asectored cell pattern is used because the cells are much closer to oneanother than in a simple cell pattern.

[0015] A first signal on a channel from a remote cell interferes with asecond (usually) stronger signal carrying a mobile transmission on thesame channel within the coverage area of a cell when the drop instrength of the first signal from the second signal is less than somethreshold level (typically measured in decibels). A signal from anothercell on a channel at a frequency adjacent the frequency of a channelcarrying a mobile transmission interferes when the drop in strength ofthe interfering signal from the serving signal is less than some secondthreshold level. The values are determined by the particular type ofmobile system involved. For example, in an AMPS system, a signal on thesame channel (co-channel) from a remote base station interferes with adesired carrier signal if the interference level is not 18 dB lower thanthe desired carrier; and a signal on an adjacent channel from anotherbase station interferes with a desired carrier signal if theinterference level is not 6 dB lower than the desired carrier. For aCDMA system, an interfering signal must be more than 14 dB stronger thanthe carrier to obscure a carrier signal because the codes establishingthe channels establish heavily redundant signals from which patterns maybe extracted even though the interfering signal is stronger.

[0016] In order to determine whether interference exists, a mobilesystem operator typically relies on customer complaints. When customersregister a sufficient number of complaints regarding communication atparticular points in a system, an operator will usually conduct arelatively expensive field test of the suspected portion of the systemto measure carrier signals and interference received. During the test,the portion of the system in which the tests are conducted isessentially disabled. Because of the expense and inconvenience, thetests are typically limited only to the suspected area. Because suchtests are limited to determining the interference at those points atwhich a system operator expects to find interference, the efficacy ofthese tests is very suspect.

[0017] The tests provide data from which the points at which channelsfrom different cells actually interfere with one another may bedetermined. If the level of interference is sufficiently large, theoperator may change the channel group assigned to the particular area.That is, the frequency group assigned to a cell (or cells) may bechanged in its entirety to another frequency group in which channelswhich would interfere with channels carried by other cells do not exist.It is also possible to eliminate some interference by changing cellcharacteristics (such as antenna tilt or power used in particular cells)without changing the channels used. Once channels have been assigned tocells which provide acceptable coverage and detected interference hasbeen eliminated, the system is fixed and operated until other complaintsarise.

[0018] A major problem with the process is that it does not provide acomplete understanding of interference which actually exists in a systemsince typically only those positions at which extensive interference hasbeen reported are tested for actual interference. The process does nottake into consideration all of the possible signals which might bepropagating into the affected area to interfere with the carrier nordoes it take into consideration the effects which a change in channelassignments may have in other areas of the system. Often (and possiblyusually) this method of curing interference merely exports theinterference to another portion of the system where it is onlydiscovered when a sufficient number of complaints arise to warrant afield test of the newly isolated area of interference.

[0019] Moreover, this method of placing cells, assigning frequencies,and eliminating interference is quite slow and labor intensive. Testinga medium sized system may require as much as 400 man hours. The processgreatly increases the costs of creating and maintaining mobile systemswithout guaranteeing that interference will be eliminated. Because ofthe emerging nature of the market for cellular telephones, systemchanges which cause interference such as traffic growth are taking placeconstantly and at an accelerating rate. Complicating the general problemof interference in an existing system is the fact that cellular systemoperators are presently installing new CDMA and TDMA systems becausethey allow a greater number of mobile units to utilize a system andbecause these digital system provides a better quality of service whenthey are functioning properly. Often the installation of these newsystems is taking place where AMPS cellular systems already exist andwill continue to exist. In general, with these systems, some of thefrequencies used in the AMPS systems are removed; and a CDMA basestation is positioned in place of a sector at a base station.

[0020] It is desirable to provide a process by which the quality ofservice provided by a cellular system (and portions thereof) may bedetermined in terms of fixed verifiable quantities so that changes maybe made to enhance the quality of service with an expectation that thechanges will have the desired result in actually improving the qualityof service provided by the system.

SUMMARY OF THE INVENTION

[0021] The present invention is realized by a computer implementedprocess which compares signals communicated between a known position anda plurality of base stations in a cellular telephone system to determinethe level of interference with a signal on a channel expected to servethe known position, and determines a value indicating a probability ofinterference with a signal on a channel expected to serve the knownposition.

[0022] In one embodiment, changes in the system to improve theinterference value are implemented only if the interference value isabove a certain level.

[0023] These and other features of the invention will be betterunderstood by reference to the detailed description which follows takentogether with the drawings in which like elements are referred to bylike designations throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a drawing depicting an idealized mobile cellulartelecommunications system.

[0025]FIG. 2 is a drawing depicting a portion of a more realistic mobilecellular telecommunications system than that illustrated in FIG. 1.

[0026]FIG. 3 is a graphical view illustrating the effect of signalsinterfering with carrier signals useful in understanding the method ofthe invention.

[0027]FIG. 4 is a flow chart illustrating a portion of a process inaccordance with the present invention in a system such as thatillustrated in FIG. 1.

[0028]FIG. 5 is flow chart illustrating another portion of a process inaccordance with the present invention in a system such as thatillustrated in FIG. 1

DETAILED DESCRIPTION

[0029] Referring now to FIG. 1, there is illustrated a cellulartelephone system 10 which includes a number of individual cells 12arranged in an idealistic honeycomb pattern. For the purpose of thisexplanation of the invention, the system 10 will be considered to be anAMPS system. This invention may be practiced, however, with any of theknown cellular systems including CDMA and TDMA systems. Moreparticularly, the signal strength data accumulated in constructing anarrow band system such as an AMPS or TDMA system may be used toconstruct or improve a CDMA or other wide band system. The dataaccumulated from an AMPS system differs from that of a CDMA system onlywith respect to the effect of Rayleigh fading; and the effect ofRayleigh fading cancels out with a sufficient number of redundant pointsof measurement. In a similar manner, the data accumulated from a CDMAsystem may be utilized to construct or improve an AMPS system.

[0030] In an AMPS system, each of the cells 12 includes at least onebase station 13 which transmits and receives communications on a numberof assigned frequencies with mobile units 15 operating within itsservice area. The frequencies which are chosen are separatedsufficiently that signals from any single base station do not interferewith other signals from that base station. In FIG. 1, the service areaof each of the ideal cells 12 is defined by an outer solid boundarywhich indicates the limits of the area in which the signals from thatcell 12 are strong enough to s serve a mobile unit 15.

[0031] As may be seen in FIG. 1, in order to allow mobile units totransmit and receive telephone communications over a wide area, theservice area of each cell 12 overlaps the service areas of a number ofadjacent cells 12 so that within these overlapping areas either of twoor more cells 12 might serve a mobile unit 15. The channels allotted tothe individual cells and the frequency reuse pattern are carefullyselected so that adjoining cells do not transmit or receive on the samefrequencies. Consequently, there are no overlapping areas over an entirecellular system in which signals of the same frequency are receivedsimultaneously from more than one cell 12 by a mobile unit 15.

[0032] In some systems, cells used in areas carrying heavy traffic arefurther divided into two or three sectors each of which may includechannels allotted as described earlier. The antennas of each threesector cell are arranged to provide 120 degree coverage. With slightlyover four hundred channels available to each cellular system, thisallows a repeating pattern of groups of cells in the beehive arrangementof FIG. 1 with seven cells each having three sectors each of which hasapproximately twenty channels.

[0033] Unfortunately, the boundaries at which the signal strength of achannel falls below a level sufficient to support communications with amobile unit vary widely from cell to cell. For this reason, cellsadjacent one another do not, in fact, typically form the precisegeometric boundaries suggested above but form a boundary patterns suchas those illustrated in FIG. 2.

[0034] Since it is necessary that each cell 12 (or sector of a cell 12if the cell is divided into sectors) have sufficient power to transmitand receive signals with a mobile unit 15 in the overlapping areas ofcell coverage to accomplish hand-off of a mobile unit transmission fromone cell to another, it is possible that channels used by differentcells will interfere with each other. As has been pointed out, channelswhich may interfere with one another are channels using the samefrequency (co-channels) and channels on frequencies immediately adjacentto a serving channel. Thus, in assigning cell sites and establishing areuse pattern, the operator attempts to assure that channels which mightinterfere with one another are not present in overlapping areas. This isrelatively simple given the ideal system such as that illustrated inFIG. 1.

[0035] However, in the more realistic system illustrated in FIG. 2, itwill be seen that areas covered by different cells overlap not onlywhere the cell sites are immediately adjacent one another but at greaterdistances. For example, coverage provided by cell 4 (in FIG. 2) isoverlapped by coverage provided by each of adjoining cells 1, 2, 3, 5,6, and 7. This overlap is normal and allows hand-off to occur when amobile unit moves from the area covered by cell 4 to any of theimmediately adjoining areas of coverage. However, coverage provided bycell 4 is also overlapped by non-adjoining cell 8. If the cells of FIG.2 are divided into sectors each covering 120 degrees, then thefrequencies of channels assigned to the overlapping areas in adjoiningcells may cause adjacent channel interference. Moreover, because of thelimited number of channels available, the sectors of cell 8 may beassigned channels which cause co-channel interference with the channelsof cell 4 in a typical frequency reuse pattern. Similar interferenceproblems exists with respect to other cells in the cellular system whichare not shown in FIG. 2.

[0036] Because the coverage offered by different cells differs sodrastically, a cellular system is usually established using softwarewhich predicts what signal strengths are to be expected from each of aparticular set of cells. This software uses input data describing thegeneral physical characteristics of the terrain surrounding eachcellular site and the physical characteristics of the cellular stationto generate estimated signal strength coverage plots for the areasurrounding a cellular site. This predictive software is used todetermine antenna positions which should provide optimum coverage withminimum interference in a typical system. However, since the predictivesoftware used to establish a system presumes general characteristicsderived from similar terrain and similar cells to determine cellcoverage, overlap such as the overlap of cell 8 into the boundaries ofcell 4 illustrated in FIG. 2 is often not predicted. In fact, it hasbeen found that the total prediction error in comparing the strengths ofthe carrier signal and interference utilizing such prior art predictivesoftware is approximately plus or minus 13.6 dB. Since a carrier signalshould be 18 dB greater than an interfering signal in order to eliminateco-channel interference in an AMPS system, this is a very largediscrepancy.

[0037] Once cell sites have been determined in some manner (e.g., usingpredictive software), the operator assigns channel groups to the cellsin accordance with the technique described above, places antennas inposition, and operates the system. Unless interference is suspected orimmediately apparent, the operator waits for subscriber complaints tosurface and then conducts physical tests at positions limited to thepositions of the complaints to determine whether interference, in fact,occurs at those positions. The determination of actual interference ismade by drive tests which measure signal strength of channels at thepositions where interference is suspected or complaints have shown thatinterference has occurred within the cellular system area. Conductingsignal to interference measurements is very labor intensive, so strengthmeasurements are typically taken only at points where interference isexpected. These tests may entirely miss interference which actuallyoccurs.

[0038] If the tests show that interference is sufficiently great at thepositions of the measurements, the groups of channels assigned to thecells having interfering channels may be changed. Determining whetherinterference is sufficiently great is accomplished by comparing at anypoint the level of interference to the signal level of the carrier.Acceptable levels have typically been chosen to be those describedabove, i.e., 18 dB for co-channel interference and 6 dB for adjacentchannel interference in an AMPS system. If interference of this level isultimately found to exist in an area which is expected to carrysignificant traffic, the frequency group assigned to a cell (or cells)is typically changed in its entirety to another frequency group whichdoes not have channels which would interfere with channels carried bythe surrounding cells. If this does not work, changing cellcharacteristics such as antenna tilt or the radiated power may eliminateinterference without changing the groups of channels used. Once channelswhich provide acceptable coverage have been assigned to cells and thepreviously detected interference has been presumed to be eliminated bythis method, the system is fixed and operated.

[0039] This operation is slow, labor intensive, and often does notprovide a complete resolution of the problem. For example, changingfrequency assignments may simply transfer interference problemsunexpectedly to other areas of the system by transferring coverage suchas that shown for cell 8 in FIG. 2 to unexpected areas.

[0040] A method has now been devised which overcomes the problems of theprior art by utilizing measured signal level data for an entire systemto provide predictive plots which may be utilized to establish cell sitepositions and channel assignments. The process allows plots and channelassignments to be easily changed at minimal cost whenever a systemundergoes change.

[0041] In one embodiment, the process begins with a drive test of theentire system area. In the drive test for this embodiment, each cell andsector transmits on a single channel different than any channel used fortransmission by any other cell or sector in the area. In general,signals on all channels transmitted from any one cell are, on anaverage, received at the same strength at any given point in the servicearea so long as the frequencies of the channels are within approximatelyten percent of each other. Thus, whatever channel a cell transmits onduring the tests, the received signal strength will be the same forsignals transmitted on any other channel from that cell.

[0042] If an entirely new system is being designed, expected cell sitesmay be selected in any of a number of different ways such as by use ofprior art predictive plotting software; and then test transceivers maybe placed at the proposed cell site positions. If a cellular systemalready exists, then the cell sites which exist are used along with anyproposed new cell sites. A mobile unit with a scanning receiver drivesover all of the roads and highways of the entire system. The mobilescanning receiver constantly scans and measures the strength (usuallyreceived signal power) of each test channel transmitted from each of thecell sites as the mobile unit moves. The mobile unit also includesequipment (such as Loran or Global Positioning System (GPS) equipment)which constantly records the position of the mobile unit as each set ofstrength measurements is taken. This provides strength measurements offrequencies generated by transmitters at all of the cell sites proposedto be included in the system which can be received at each point in theservice area over which the mobile unit drives. By transmitting fromeach cell on a single different channel, the cell which is transmittingany signal received at any point by the mobile unit is positively known.As the test continues, the signal strength measurements of all signalsreceived (or all signals greater than a certain level) are recorded in adatabase by equipment in the mobile unit together with the position atwhich the signals were received.

[0043] It should be noted that certain interference, typically Rayleighfading, is essentially intermittent in nature. Such interference tendsto strengthen and weaken received signal strength over very shortdistances. In order to eliminate the effect of this intermittent fading,readings may be taken at a number of positions quite close together andlater averaged in order to provide quite an accurate representation ofthe strength of signals received at any point. In one embodiment, eachdata sample is combined with other data samples within one hundred feetof each other to eliminate the intermittent effects and normalizesamples taken during different test drives. Since Rayleigh fading is theprimary difference between received signal strengths in different typesof mobile systems, the data gathered from tests conducted in narrow bandsystems may be used in the design or improvement of wideband systems.

[0044] The frequency of each piece of signal strength data in thedatabase is then related to the test channel being transmitted by eachcell and sector during the test. This generates a database whichindicates the cell and sector from which each signal received by themobile unit was sent. The cellular strength data base thus includesactual, rather than projected, received signal strengths at each pointin the test area for signals transmitted from each cell.

[0045] It should be noted that the signal strength data for an area canbe compiled from more than a single drive test. In such a case, the datafrom all of the drive tests must be combined so that the data of eachdrive test matches that of other drive tests. Thus, for example, ifhigher transmission power was used in one drive test than in another,then the strength values should be scaled to provide data having thesame significance. The data collected from one drive test may also be“combined” with previously collected data from other drive tests if thenew data represents only a portion of the cells in the network. Ofcourse, if data is already available from previous wide area testdrives, then this data may be used and no test drives need be conducted.This step is useful when adding new cells to a network so that theeffect of new cells may be determined without having to re-collect datafor the entire network.

[0046] A second method of collecting signal strength data providessubstantial economies over the method explained above, especially whennew sites are being planned and a particular site has not yet beenselected. Tests have shown that the signal strength received at a cellsite from the mobile transmitter in an uplink transmission is on anaverage the same as the signal strength which would be received at amobile unit from a cell site in a downlink transmission. If the uplinkand downlink signal strengths differ, comparable values may be obtainedby adjusting the amplifications and power values. Thus, rather thanconducting drive tests with transmitters placed at each proposed cellsite as in the first method and checking each against the other, drivetests are conducted by placing a single transmitter in a mobile unit andusing fixed receivers (rather than expensive scanning receivers) at allof the proposed positions at each of the sites over an area for whichnew cells are proposed. The mobile unit drives over the roadsencompassed by the new cells transmitting on a single frequency whileall of the receivers attempt to detect the transmission. The power leveltransmitted by the mobile antenna is measured at the mobile unit, and apositioning system is linked to the mobile unit to provide positionindications at each point of measurement. The mobile transmitter sends asignal at the selected frequency, and the receivers at all of the cellsmeasure its strength. The position of the mobile unit for each of thetest transmissions is recorded with the times of the transmissions in adatabase. The signal strength received at each proposed site and thetimes of reception are recorded by each receiver. Since the signalstrength received at a cell site from the mobile transmitter in anuplink transmission is on an average the same as the signal strengthwhich would be received at a mobile unit from a cell site in a downlinktransmission (or may be adjusted to be so), the data gathered by thedrive test using this second method may be directly substituted for thedata gathered in the drive tests for the previous method.

[0047] Once the data is available, however it has been collected, theprocess compares the data for each channel received at each point in theentire area with the data for all other channels received at the pointto determine at any point which cells should serve the point. Thesecells are called “likely servers.” A number of criteria may be used.

[0048] In general, a cell is a likely server at a particular location ifthere is a non-trivial probability that a cell will provide atransmission path to or “serve” a mobile unit at that location.Different methods may be used to determine likely servers. A basicmethod identifies as likely servers all cells that serve a location witha signal strength within 3 dB (or some other value depending on thesystem) of the strongest signal strength for that location. Moresophisticated methods may account for signal path imbalances, maybalance the uplink and downlink strengths where they vary, may biascertain strength determinations in favor of particular cells, or provideother adjustments to match the particular area of the system. The methodmay also account for each different type of network hardware and networkconfiguration and control information (e.g. how mobile unit hand-off isperformed) to determine likely servers for each location.

[0049] Using the basic method, the cell providing the strongest signalat a point is typically designated the cell to serve that point becausesignals on any channel on which the cell transmits will be received atapproximately the same signal strength. Signals on other channelsreceived at the same point but at lesser strengths still within the 3dB. range typically are transmitted by adjoining cells in whatconstitutes a hand-off (overlap) area for that point. The service areafor each such cell is ultimately determined by applying the plannedpower, path imbalance, and handoff parameters to the test data which hasbeen accumulated.

[0050] Once the cells serving all of the points of a service area areknown, the group of channel proposed for each of the cells or sectors isassociated with those cells. When the channels for each cell are known,the signal strength provided by each cell which is the server at eachtest position in the cellular system is compared with the signalstrengths of all cells transmitting signals received at each testposition which transmits on channels which could cause co-channel oradjacent channel interference. This allows a determination of whetherthe proposed channel selection causes either co-channel or adjacentfrequency interference at any point in the system. Since the points atwhich signals on any particular channel transmitted by one cell willhave a certain strength and may interfere with signals from another cellmay be determined from the signal strength data which has beencollected, such a determination may be made for each proposed point andchannel in the system. Whether a signal will interfere is usuallydetermined by subtracting the interfering signal strength in dBm fromthe signal strength of the carrier signal serving the point in dBm ateach point. The cells which are likely servers at each point havealready been determined from the test to determine cells serving apoint. For co-channel interference in the AMPS system, if the differenceis less than 18 dB, interference exists. For adjacent channelinterference in the AMPS system, if the difference is less than from 3to 6 dB. (depending on the criteria used), interference exists. If thereis interference at any point in the system, the pattern of channelassignments and other cell configuration information (such as effectiveradiated power (ERP)) may be changed; and the actual signal strengthdatabase may be run against the new cell channel assignments. Thisrequires no new testing or other operations by the operator; it requiressimply running the software until channel selections which excludeinterference are determined.

[0051] Not only may the process be used to update or plan a new system,the process also allows signal strength measurements derived from drivetests conducted using a particular type of cellular system such as anAMPS to be used for determining coverage and interference patterns forcell sites utilized by entirely different types of systems. This has theadvantage of allowing drive test results accumulated from an oldersystem to be used to predict interference which may occur in newer typesof systems which might be installed at the same sites. The same signalstrength test results may be utilized as a system is changed in anymanner. In a similar manner, if an operator has already established CDMAchannels from which the strength of signals may be discerned, it ispossible to use this data to optimize the performance of the AMPSchannels which exist at the same cell site. An additional benefit isthat the CDMA measurement process is non-invasive so that the operatordoes not have to “key-up” channels for testing to derive data.

[0052] In an AMPS system, the new channel assignments may be tested bythe software against the signal strength measurement database to derivenew predictions of interference. If additional cells or sectors are tobe added, this may be accomplished by drive tests for signals from thenew cells only. These may be added to the signal strength measurementdatabase and the updated database used to determine new channels to beused.

[0053] It has now been determined that this process may be madesubstantially more useful by modifying the process to provide consistentvalues which indicate just how the various points, sectors, and cells inthe system, and the system itself compare with other points, sectors,cells, and systems. Such a value is more readily understood by systemoperators and allows changes to be planned with an understanding of theresult which will be accomplished by those changes.

[0054] In order to generate values which have meanings which remainconsistent wherever they are determined, the improved process relatesnot only the strengths of carrier signals and signals which interferewith those carrier signals but also determines the probability ofoccurrence of the various interfering signals and the severity of theinterference during receipt of the interfering signal. This allows aninterference value to be determined which essentially indicates thepercentage of time a subscriber to a mobile system may expect toencounter perceptible interference at any point in the system. Moreover,the interference values for points within a sector, cell, and system maybe accumulated and averaged in the manner described in FIG. 4 to providean interference value for sectors, cells, and the system. This allows anoperator to pinpoint sectors and cells which need to be improved andprovides an overall evaluation of a system from which an operator maydetermine rationally whether improvements need to be made. Using theinterference values for points in a system, the efficacy of each changeto the system may be evaluated as it is proposed. Each type of changewhich might be made may be compared to other types of changes in orderto make the most economical changes possible.

[0055] To understand how a consistent interference value may be derived,the process of interference has been dissected to determine itselements. For example, if it is possible that three different signalsmay interfere with a particular signal from a base station which is amost likely server, then the actual likelihood of each of these signalsinterfering can be considered in order to better understand how receiptof signals at that point compare with receipt of signals at other pointsand thus to have an idea on how to improve a system. This isaccomplished by the use of a probability number assigned to each of thedifferent interfering signals determined from the traffic patterns andother factors known (or estimated) to occur for the particular basestations. A cell in an area having more traffic transmits during agreater portion of the time spectrum.

[0056]FIG. 3 illustrates a plot of co-channel interference ratios(carrier strength of signal from a primary server divided by signalstrength of a co-channel signal received) versus the effect those ratioshave on transmission of a carrier signal in an AMPS system. The effectis shown as a weight value which indicates the severity of theinterference. As may be seen, if the co-channel interference is greatenough so that the difference in signal strength is less thanapproximately 10 dB, then the interference is too great for any usefultransmission. Such an interference level is given a weight of one. Onthe other hand, if the signal strength of a carrier signal is more than18 dB greater than the signal strength of the interfering co-channel,then the effect on the transmission is nil; and a weight of zero isgiven. Between these values, the interfering signal has greater andlesser effects as may be seen from the figure.

[0057] In one embodiment of the invention, the presumption is made thatif two or more signals may possibly interfere with a carrier at anypoint in the system, the effect of the stronger interfering signal willnegate any effect that the weaker signals may have during time thestronger signal is being received. Although this is an approximation,its use has little affect on the accuracy of the results produced. Theuse of this presumption means that only the stronger interfering signalneed be considered at any time. Thus, to determine the overall effect ofthree interfering signals, the probability of the occurrence of eachsignal is determined and then multiplied by the weight value todetermine the effect that signal has. For example, a strongestinterfering signal within 10 dB of the carrier has the weight one(indicating that the carrier signal is entirely obscured duringtransmission of the interfering signal) multiplied by the probability ofoccurrence. Thus for the 2 dB signal shown in FIG. 3, its probability is0.4; and its effect is obtained by multiplying this probability by theweight of one.

[0058] Once the effect of the strongest interfering signal has beendetermined, its probability of transmission is subtracted from one toprovide the probability that the first interfering signal is not active.The result of this computation provides the time range within which thesecond strongest interfering signal occurring will have significanteffect. Thus, the to probability that the second strongest signal of 12dB will interfere is the probability factor 0.6 of the second signaloccurring multiplied by the time during which it will have significantinfluence (0.6 of the total time). This probability for the secondsignal is multiplied by its weight of 0.84 to determine its effect. Theprobability that the third signal of 15 dB will interfere is determinedby multiplying the probability that the first interfering signal is notactive by the probability that the second interfering signal is notactive by the probability factor for the third signal occurring. Thisprobability factor is then multiplied by the weight of 0.32 for thethird signal to reach an effect for the third signal.

[0059] Adding the effect of all of these signals interfering provides afinal result of 0.7408 which may be stated as a percentage and providesa quality number for the particular point in the system with the plannedchannels and parameter settings. In essence, the interference valueindicates the percentage of time interference will be present at thepoint. Obviously, the value of 74% indicates that receipt of signals atthe particular point is almost impossible. This interference or qualityvalue may be compared with interference values for all other points in aservice area.

[0060] Once a quality value for a point has been obtained, qualityvalues are obtained for some number of additional points in the sectorsufficient to provide a relatively good evaluation of all of the placesin the sector at which communications may be received. The qualityvalues obtained for a sector are then added together and divided bytheir number to obtain an average quality value for the sector (orcell). FIG. 4 illustrates the method by which this is accomplished,finding first an interference value for a point, then a next point, andso on until the points for a sector are all determined. Then all sectorsvalues are determined and finally a sector score is reached.

[0061] Similarly, once the quality value for one sector has beenobtained, quality values for all sectors in a system may be similarlyobtained, added together and averaged to provide a quality score for theentire system. This score may then be utilized to determine whether thesystem should be changed in order to provide improved service. Utilizinga quality value which is consistently applied from point-to-point,sector-to-sector, and system-to-system allows a valuation to be madefrom which some real determination of quality may be made.

[0062] More specifically, if a quality valuation for a sector is known,it is possible to determine whether changes which might be attempted inthe system would be successful. That is, different changes to aparticular sector may be assigned different quality increments bytesting to determine the effect those changes might have. For example,changing the power level of an interfering signals from another sectorcan clearly be ascribed an increment since the level of signal receivedis an exact value in reaching the original interference level. With anincremental value to be applied for a change to the sector, it may beknown before any change is made whether that change will provide animprovement in the sector and system quality.

[0063]FIG. 5 is a flow chart which illustrates the operation of themethod to improve the quality of a system once a quality value for thesectors and a system are found and the values of possible changes areknown. As may be seen, the method begins with the original interferencevalue for a point, and selects a best change to improve the quality ofservice for the point. Often when beginning to improve a system, thisbest change is a change in the group of frequencies assigned to one ormore sectors (or cells). Probably the next change to be made onceappropriate frequency groups have been chosen, is to change powersettings of transmitters. Biasing the level of handoff so that thehandoff occurs if two channels are within two, three, or four dB of oneanother in a handoff area changes the point at which handoff occurs andthe level of power necessary in those areas. Other changes which arepossible include changing antenna types, and other changes related toequipment modification.

[0064] The method illustrated in FIG. 5 may be used in more than onemanner. It may be used to iterate through changes of one type (e.g.,change the frequency assignments) computing each result as it isimplemented in software until an interference value is reached which isthe best that that form of change can accomplish. Alternatively, it maybe used to select among different types of changes to determine whichprovides a better result when compared to the cost of implementing thechange.

[0065] Presuming for the moment that the change is an iteration througha selection of possibilities of one type (frequency groups) until a bestresult is reached, a list of changes is prepared by comparing theinterference levels at each point to determine which frequenciesinterfere with one another. A particular change is selected from thelist of possible changes, and a determination is made by running thesoftware whether the change produces a result greater than some value sothat the change is worth undertaking for the improvement to be expected.When testing different frequency groups, the change making the processworth while may be a reduction of some percent (e.g., one percent) inthe interference value. Changing frequency groups, changing powerlevels, or biasing the handoff level differently usually costs nothingbut processing time and is worth while if it produces a concrete result.Other changes may require new equipment and be more expensive, however.

[0066] If the change contemplated does not produce an improvementsufficient to warrant its use, the change may be thrown out and a finalinterference value determined. If the change is worth making, the listof changes is updated to show that the particular change has beenevaluated and the amount of change is listed in a list of changes. Thechange is added to a list of changes to make as a best change if it isthe first or best tested. It is also listed as the best change to make.The process then iterates through the list and for each change above theminimum change which is worth while, updates the list of possiblechanges by removing the tested changes from the list of those changesstill to be tested, recording the change value, adding the change to theoptimization steps if its effect is greater than preceding changes, andreplacing the best change with the latest change if the result iscorrect. Ultimately, the best change to be made for the particular pointis reached. A similar process occurs for all other points in the system.Ultimately, a result for changing the particular factor that producesthe best result for each sector and the system is reached.

[0067] The method may then proceed with any other changes which mightimplemented to improve the system. The same iterative method may be usedto determine a best change of the particular type for each point,sector, cell, and the system.

[0068] Alternatively, different types of changes may be given differentweightings and the entire process carried out for each point withrespect to all of the possible changes to determine which changes shouldbe implemented to produce the best results.

[0069] Although the present invention has been described in terms of apreferred embodiment, it will be appreciated that various modificationsand alterations might be made by those skilled in the art withoutdeparting from the spirit and scope of the invention. The inventionshould therefore be measured in terms of the claims which follow.

What is claimed is:
 1. A computer implemented process comprising thesteps of: comparing signals communicated between a known position and aplurality of base stations in a cellular telephone system to determinethe level of interference with a signal on a channel expected to servethe known position, and determining a value indicating a probability ofinterference at the known position.
 2. A computer implemented process asclaimed in claim 1 in which the step of determining a value indicating aprobability of interference at the known position includes the steps of:combining for each signal being received at the known position aprobability of receipt of such signal and a weighting indicating theseverity of interference to be expected from such signal at the knownposition to determine an effect for such signal, and combining theeffects for all signals being received at the known position todetermine the value indicating a probability of interference at theknown position.
 3. A computer implemented process as claimed in claim 2in which the step of combining the effects for all signals beingreceived at the known position includes the effect of only a strongestinterfering signal during any interval.
 4. A computer implementedprocess as claimed in claim 1 comprising the additional step ofaveraging values indicating a probability of interference at knownpositions within a communication area served by one of the base stationsto determine a value indicating a probability of interference within thecommunication area served by the base station.
 5. A computer implementedprocess as claimed in claim 3 comprising the additional step ofaveraging values indicating a probability of interference at knownpositions within a communication area served by one of the base stationsto determine a value indicating a probability of interference within thecommunication area served by the base station.
 6. A computer implementedprocess as claimed in claim 4 comprising the additional step ofaveraging values indicating a probability of interference within thecommunication areas served by the plurality of base stations todetermine a value indicating a probability of interference within thecommunication area served by the cellular telephone system.
 7. Acomputer implemented process as claimed in claim 5 comprising theadditional step of averaging values indicating a probability ofinterference within the communication areas served by the plurality ofbase stations to determine a value indicating a probability ofinterference within the communication area served by the cellulartelephone system.
 8. A computer implemented process as claimed in claim1 in which the signals are determined from actual field tests.
 9. Acomputer implemented process as claimed in claim 8 in which the signalsare determined from actual field tests of cellular systems establishingchannels on a basis different than the basis of the cellular telephonesystem.
 10. A computer implemented process comprising the steps of:combining values indicating strength of signals communicated betweeneach of a plurality of closely adjacent known positions and a pluralityof base stations in a cellular telephone system to determine averagestrengths of signals communicated between an average known position andthe plurality of base stations, comparing the average stength of signalscommunicated between an average known position and a plurality of basestations in a cellular telephone system to determine the level ofinterference with a signal on a channel expected to serve the averageknown position, and determining a value indicating a probability ofinterference at the average known position.
 11. A computer implementedprocess as claimed in claim 10 including the further steps of: selectinga projected change to implement which affects strength of a signalbetween the known position and the plurality of base stations, anddetermining the improvement in the value indicating a probability ofinterference at the average known position by implementing the projectedchange.
 12. A computer implemented process as claimed in claim 11including the further steps of: selecting additional projected changesto implement which affect strength of a signal between the knownposition and the plurality of base stations, and determining theimprovement in the value indicating a probability of interference at theaverage known position by implementing the projected change until theimprovement is less than a predetermined value.
 13. A computerimplemented process as claimed in claim 10 in which the valuesindicating strength of signals are values determined from actual fieldtests.
 14. A computer implemented process as claimed in claim 13 inwhich the values indicating strength of signals are values determinedfrom actual field tests of cellular systems establishing channels on abasis different than the basis of the cellular telephone system.
 15. Acomputer implemented process comprising the steps of: collecting dataindicating the actual strengths of all signals to be transmitted betweena plurality of cells each positioned at an individual physical positionin a mobile communications system and a mobile unit at a plurality ofpoints defining an entire mobile communications system, comparing actualstrengths of all signals serving a point from each of the plurality ofcells with all other signals received by a mobile unit at each point ofthe system to detect signals other than desired signals which rise toestablished levels of interference for the particular system,determining a value which measures a level of interference at each pointin the system, utilizing the values to determine values which measurethe level of interference for each cell and the system, determiningwhether any value which measures the level of interference for each cellis sufficient to warrant reducing interference in the cell, andselecting corrections in cell characteristics which reduce signalstrength of interfering signals until interference has been reducedbelow a predetermined interference level throughout the system.